1. Field of the Invention
This invention relates to a process for preparing monodispersed silver halide photographic emulsion and more particularly to a process for preparing a monodispersed silver iodobromide photographic emulsion.
2. Description of the Prior Art
Recently there have been increasing demands for silver halide photographic emulsions having better photographic characteristics such as high sensitivity, excellent graininess, high sharpness, low fog density and sufficiently high optical density.
Silver iodobromide emulsions containing iodide of from 0 to 10 mol% are well known as to high-sensitive emulsions capable of satisfying the above-mentioned demands. As for the processes of preparing these emulsions, there are conventionally known an ammonia method, a neutral method, and an acid method which control pH and pAg conditions; a single jet method and a double jet method which are called a method of mixing. Based on these well-known techniques, and in order to further attain high sensitivity, improved graininess, high sharpness and low fog, various technical means have been studied in detail and put to practical use. Silver iodobromide emulsions, which are the subject of the invention, have been so studied as to control not only a crystal habit, grain size distribution, but iodide density distribution in each of silver halide grains.
The most orthodox method to accomplish the above-described photographic characteristics such as high sensitivity, excellent graininess, high sharpness, low fog density, sufficiently high convering power is to improve quantum efficiency of a silver halide. For this purpose, the observation of solid state physics is positively introduced. The study of theoretically calculating this quantum efficiency and examining the influence of grain size distribution is described, for example, in "Interactions Between Light and Materials for Photographic Applications" on page 91 which is a preprint for 1980 Tokyo Symposium on photography advancement. According to this study, it is predicted that quantum efficiency will be improved with a monodispersed emulsion prepared by narrowing the range of grain size distribution. And further, it is considered to be reasonable to infer that a monodispersed emulsion may be advantageous for sensitizing a silver halide emulsion as well as for efficiently attaining high sensitivity with keeping fog lower in a chemical sensitization process which will be described in detail later.
For industrially preparing a monodispersed emulsion, as is described in Japanese Patent Publication Open to Public Inspection No. 48521/1979 (hereinafter referred to as Japanese Patent O.P.I. Publication), it is necessary to control the conditions of supply speeds of silver ions and halide ions to the reaction system which is calculated theoretically and the conditions of sufficient agitation, under severe control of the pAg and pH values thereof.
It is known that silver halide grains can variously be formed according to the controls of pAg during the growth of silver halide crystals. Silver halide emulsions prepared under these conditions have a shape of a cube, octahedron, or tetradecahedron respectively and consist of normal crystal grains having (100) face and (111) face in various proportion.
Measurement and control of pAg are monitored and controlled by means of devices already known in the art. Typical and useful control devices are described in U.S. Pat. No. 3,031,304 and photographische Korrespondenz, 1967 vol. 103, pp. 161-164.
However, it is rather difficult to practically prepare these monodispersed emulsions. As for a process of preparing monodispersed iodobromide emulsions, there are known those described in Japanese Patent O.P.I. Publication Nos. 49938/1983 and 157636/1982. Even such processes have a tendency that grain size distribution widens remarkably when silver iodide contents increase. That is, even with pure silver bromide crystals and under preparation conditions for good monodispersibility, when containing 2 to 40 mol% of silver iodide, the monodispersibility will be deteriorated. This tendency gets more remarkable with the increase of silver iodide content.
For example, according to a preparation process disclosed in Japanese Patent O.P.I. Publication No. 157636/1982, it is possible to prepare a monodispersed emulsion containing about 2 mol% of silver iodide, but when the content of silver iodide becomes 4 to 8 mol%, the monodispersibility deteriorates remarkably. This tendency is shown particularly in the grains in from tetradecahedral to octahedral shape rather than in the grains in the cubic shape.
High sensitive negative photographic materials of common type require to contain generally 2 to 10 mol% of silver iodide, and the so-called core-shell type emulsion which has a portion of higher silver iodide content within a grain needs for its core, even if it contains grains of 2 to 10 mol% silver iodide, monodispersed silver iodobromide grains containing 10 to 40 mol% silver iodide. Therefore, strongly desired is a technique to overcome the above-mentioned characteristics that monodispersibility deteriorates with the increase of silver iodide.
On the other hand, there are conventionally known preparation processes in which some additive is used in the process of forming silver halide crystals. For example, Berichte der Bunsengesellschaft, 1963 vol. 67, pp. 349-355, describes that the formation of silver chloride crystals may be stopped by adding a purine base such as adenine in the various stages of forming silver chloride crystals. U.S. Pat. No. 3,519,426 describes that fine grains of silver chloride may be formed by adding azaindene such as tetrazaindene, pentazaindene and adenine when silver chloride is precipitated and thus covering power is improved.
Further, Japanese Patent O.P.I. Publication No. 111937/1983 describes that in the formation of fine crystals of silver chlorobromide containing not less than 50 mol% of silver chloride, fine crystal of silver chloride in tablet shape may be obtained by using peptizers having a bond of aminoazaindene and thioether.